Promethium sources

ABSTRACT

There is provided an apparatus and a method for the uniform deposition of the Beta emitting nuclide 147 Pm onto precisely defined areas. The technique both produces and deposits the emitting materials simultaneously without the need for separate preparation and purification of the metallic promethium.

United States Patent Parry et al.

PROMETHIUM SOURCES Inventors: Colin Parry; Kenneth J. Round,

both of Ottawa, Ontario, Canada Assignee: Atomic Energy of CanadaLimited,

Ottawa, Ontario, Canada; a part interest Filed: June 16, 1969 Appl. No.:833,797

Foreign Application Priority Data Field of Search ..117/107, 71, 220,217, 227

[451 Jan. 16, 1973 [56] References Cited UNITED STATES PATENTS 3,096,2117/1963 Davis ..117/107 X 3,489,593 l/197O Cohen et a1 ..1 17/107 XPrimary Examiner-Alfred L. Leavitt Assistant ExaminerC. K. WeiffenbachAttorneyCushman, Darby & Cushman [57] ABSTRACT There is provided anapparatus and a method for the uniform deposition of the [3 emittingnuclide Pm onto precisely defined areas. The technique both produces anddeposits the emitting materials simultaneously without the need forseparate preparation and purification of the metallic promethium.

12 Claims, No Drawings PROMETHIUM SOURCES This invention relates to amethod and apparatus for promethium deposition. The invention hasparticular but not exclusive use in the preparation of promethium betasources for use in nuclear batteries, thickness gauges, luminoussources, etc., although such sources will find many other applicationsto those skilled in the art.

It is well known to deposit elements or mixtures of elements by vacuumevaporation and there is abundant literature covering the subject.However, it has not previously been known to produce and simultaneouslydeposit a thin film of radioactive metal in a single operation, Thenovel process described herein has certainly never heretofore beenapplied to the production of promethium sources.

In many applications for beta emitting radio nuclides at least one ifnot all of the following characteristics must be considered:

a. The beta emitting material must be spread in a very thin uniformlayer on the supporting substrate which, in the case of nuclearbatteries must be electrically conductive.

b. The radioactive material must be bonded to the substrate, otherwiseit may fall off in use, or in the case of nuclear batteries, beattracted to an associated collector electrode.

c. The radioactive material usually needs a thin protective coating topreserve it from mechanical abrasion or damage, and also in the case ofreactive elements to prevent tarnishing when exposed to air. Thiscoating must be as thin as possible and is desirably made of low massnumber material to minimize beta particle absorption.

The present invention envisages the deposition of the beta emittingnuclide Pm. This material is characterized by a low energy beta emissionand its relative cheapness and in view of these it is attractive for usein nuclear batteries. Being a rare earth element, however, it haschemical properties which preclude the usual simple methods of thin filmsource preparation such as electro-deposition.

It is a feature of one aspect of the invention to provide a method forthe simultaneous production and deposition of promethium.

in accordance with the foregoing object the method comprises the stepsof (i) preparing a mixture ofa compound of a beta emitting nuclide ofpromethium and a reducing agent, (ii) heating said mixture under vacuumto a temperature sufficient to effect reduction of said compound andevaporation of the promethium; (iii) depositing the promethium vapor ona substrate to form a thin uniform layer of the metal, and (iv) sealingsaid promethium deposit by the evaporation and deposition of a low massnumber metal or other suitable substance before exposure of saidpromethium to the atmosphere.

The basic aspect of the invention is to evaporate promethium undervacuum, and to deposit the vapor onto a suitable substrate. The freshlydeposited promethium is then sealed and protected by a subsequent thindeposit of a vapor of a low mass number metal such as aluminum ortitanium before the promethium is exposed to the atmosphere.Alternatively, a thin layer of various compounds such as silica,

glass, titanium oxide, alumina, or other ceramic materials may be usedfor same purpose.

The starting material is normally promethium oxide Pm O but other formsof promethium, such as promethium fluoride (PmF may be used. The oxideis intimately mixed with an excess of a suitable reducing agent, placedin crucible, and heated to a high temperature, in the approximate range(l600 to 2000 C) and enclosed in a chamber under a vacuum. Best resultsare obtained with vacuums of 10 mm of mercury or better, although theprocess works with reduced efficiency at 10 mm of mercury. In ourprocess a tantalum crucible was used since tantalum is compatible withrate earth metals at high temperature. Other crucible materials mightwork and limited success was achieved with molybdenum.

The promethium metal is produced by a reaction, such as The reducingagent M may be any metal for which reaction (1) can proceed. However, itis desirable that the reducing agent and its oxide be less volatile thanthe promethium at the temperature of reduction, to avoid contaminationof the deposited layer. Lanthanum has been found satisfactory as areducing agent, since lanthanum metal and lanthanum oxide both have lowvolatilities. Preferential evaporation of promethium will aid inshifting the chemical equilibrium of reaction (1) in the desireddirection to the right.

The tantalum crucible may take the form of a hollow metal container witha small hole in the wall thereof. Our crucible had a cylindricalconfiguration. The crucible may be heated by any convenient means,although we found that the passage of a heavy electric current throughthe crucible was satisfactory. The temperature must be observed and anoptical pyrometer proved satisfactory.

The promethium metal formed in the reduction process evaporates andpasses out through the hole in the crucible. A carefully cleanedsubstrate placed close to the crucible, intercepts the stream ofpromethium vapor, which forms a uniform film of the desired thinness onthe substrate.

The substrate is mounted within the vacuum chamber and is mechanicallymovable between a first position where the promethium coating isdeposited to a second position in which a layer of aluminum, titanium,or other suitable material is subsequently deposited. The aluminum isusually evaporated from an electrically heated tungsten filament, but itcan also be evaporated from a suitable refractory crucible. Titanium isusually evaporated from a tungsten filament or from an electron beamheated source. Ceramics may be evaporated from suitable crucibles ordeposited by RP. spluttering. Perforated metal masks may be placedbetween the evaporation sources and the promethium coated substrate tocontrol the area of deposition, and to ensure that the aluminum or otherover-coating overlaps the edges of the promethium. The low mass numbermaterials may also comprise beryllium, nickel, chromium, glass, siliconoxide, silicon nitride, aluminum oxide or titanium oxide.

Our invention has several advantages which include:

a. Elimination of the need to separately prepare and purify metallicpromethium, which is a difficult operation'in view of the chemistry ofthe element and its rarib. The process gives a very thin and uniformlayer of promethium on the substrate. The promethium metal film alsoappears to form a tenacious bond to metal substrates.

c. The over-coating gives good protection from atmospheric attack andmechanical abrasion during battery assembly, but is so thin and of suchmaterial that that beta particle emission is only slightly attenuated.

d. The process may be used to deposit promethium on precisely definedareas of almost any substrate material which will withstand the radiantheating of the evaporation sources.

e. With the apparatus we have developed, substrates may be coated onboth surfaces. It is also possible to coat many sources in a singleoperation, the present apparatus, for example, can handle 24 substratespecimens at a time. Large scale productions process should berelatively simple.

While the invention has been described with application to the nuclidePm, the process is applicable to other rare earth elements. whosechemistry is similar to promethium. The process should work well for anyreasonably volatile metallic element which can be reduced in situ.

Other variations and applications falling within the terms of theappended claims will occur to those skilled in the art.

We claim:

1. A method of preparing a radioactive metal comprising promethium betasources and simultaneously depositing a relatively thin, uniform filmthereof on a support-substrate in a single operation comprising thesuccessive steps of:

a. mixing a source of beta emitting nuclide of promethium with areducing agent therefore;

b. heating said mixture at a temperature of about l,600 to about 2,000C. under a reduced pressure of at least mm of mercury, said temperaturesufficient to effect:

1. reduction of said promethium source, and 2. evaporation of saidpromethium;

c. deposting the promethium vapor formed in step (b) on a substratethereby forming a thin uniform layer of promethium metal thereon; and

d. sealing said promethium deposited in step (c) before exposure of saidpromethium layer to the atmosphere'.

2. Method according to claim 1 wherein said reduced pressure is fromabout 10' to 10" mm of mercury.

3. A method of preparing thin promethium beta sources comprising thesteps of:

i. preparing a mixture of a reducing agent and a compound of a betaemitting nuclide of promethium selected from the group consisting ofpromethium oxide and promethium fluoride,

ii. heating said mixture in a vacuum to a temperature sufficient toeffect reduction of said compound and to effect the formation ofpromethium vapor,

iii. depositing said promethium vapor on a substrate to form a thinuniform layer of promethium metal thereon, and IV. sealing thepromethium layer by the evaporation and deposition of a layer of sealingmaterial to protect said promethium layer from atmospheric attack andmechanical abrasion.

4. The method defined in claim 3 wherein the heating of step (ii) iseffected by the passage of an electric current.

5. The method defined in claim 3 wherein said sealing material in step(iv) is selected from the group consisting of aluminum, titanium,beryllium, nickel, chromium, glass, silicon nitride, aluminum oxide ortitanium oxide.

6. The method defined in claim 3 wherein step (iv) is effected by theelectrical heating and vaporization of aluminum.

7. The method defined in claim 3 wherein said sealing material istitanium and wherein the titanium is evaporated from a heated filamentor from an electron beam heated source.

8. The method of claim 3 wherein said reducing agent and its oxide isless volatile than the said promethium at the temperature of reduction.

9. The method of claim 3 wherein said reducing agent is lanthanum metal.

10. The method defined in claim 3 wherein said nuclide of promethium isPm.

I 11. The method of claim 3 wherein the promethium vapor produced instep (ii) is in the form of a stream.

12. The method defined in claim 11 wherein said stream is directedtowards said substrate when said substrate is in a first position andthe deposition of said sealing material is effected when said substrateis subsequently moved to a second position.

2. evaporation of said promethium; c. deposting the promethium vaporformed in step (b) on a substrate thereby forming a thin uniform layerof promethium metal thereon; and d. sealing said promethium deposited instep (c) before exposure of said promethium layer to the atmosphere. 2.Method according to claim 1 wherein said reduced pressure is from about10 5 to 10 4 mm of mercury.
 3. A method of preparing thin promethiumbeta sources comprising the steps of: i. preparing a mixture of areducing agent and a compound of a beta emitting nuclide of promethiumselected from the group consisting of promethium oxide and promethiumfluoride, ii. heating said mixture in a vacuum to a temperaturesufficient to effect reduction of said compound and to effect theformation of promethium vapor, iii. depositing said promethium vapor ona substrate to form a thin uniform layer of promethium metal thereon,and iv. sealing the promethium layer by the evaporation and depositionof a layer of sealing material to protect said promethium layer fromatmospheric attack and mechanical abrasion.
 4. The method defined inclaim 3 wherein the heating of step (ii) is effected by the passage ofan electric current.
 5. The method defined in claim 3 wherein saidsealing material in step (iv) is selected from the group consisting ofaluminum, titanium, beryllium, nickel, chromium, glass, silicon nitride,aluminum oxide or titanium oxide.
 6. The method defined in claim 3wherein step (iv) is effected by the electrical heating and vaporizationof aluminum.
 7. The method defined in claim 3 wherein said sealingmaterial is titanium and wherein the titanium is evaporated from aheated filament or from an electron beam heated source.
 8. The method ofclaim 3 wherein said reducing agent and its oxide is less volatile thanthe said promethium at the temperature of reduction.
 9. The method ofclaim 3 wherein said reducing agent is lanthanum metal.
 10. The methoddefined in claim 3 wherein said nuclide of promethium is Pm147.
 11. Themethod of claim 3 wherein the promethium vapor produced in step (ii) isin the form of a stream.
 12. The method defined in claim 11 wherein saidstream is directed towards said substrate when said substrate is in afirst position and the deposition of said sealing material is effectedwhen said substrate is subsequently moved to a second position.